1. Field of the Invention
The present invention relates to an image forming apparatus capable of forming images on a photoconductor formed by laying a photoconductive material having a proper photoconductivity on a conductive base.
2. Description of the Related Art
The inventors of the present invention know that there is an image forming apparatus (hereinafter, it is called a copying machine) capable of forming an image by using a photoconductor having a proper photoconductivity.
In general, the photoconductor is formed by laying a photoconductive layer made of a photoconductive material on a photoconductive base made of aluminum (Al), in a shape of a drum or a belt and mounted inside of the copying machine.
An image forming part of the copying machine consists of a charger, an optical system device, a developer, a transfer device, a cleaner, a discharger, and they are arranged around the photoconductor to carry out the following processes in order;
(a) a process of uniformly charging the surface of the photoconductor. PA1 (b) a process of forming a latent image by a radiation of light reflected by a document. PA1 (c) a process of forming a toner image by sticking toner onto the latent image. PA1 (d) a process of transferring the toner image onto a sheet. PA1 (e) a process of removing residual toner from the surface of the photoconductor. PA1 (f) a process of removing residual potential.
As the above-mentioned charger, a scolotron charger is well known and widely used.
The scolotron charger is formed of a metallic grid or a fine wire which is disposed between a corona wire and a photoconductor and enables to charge the photoconductor with a stable potential.
A corona current flowing in the corona wire is controlled by applying proper voltages to the grid.
A life of the photoconductor is determined according to the thinning degree of the photoconductive layer since the performance of the image forming apparatus will be lowered on a basis of the thinning of the photoconductive layer, as a consequence, the numbers of the formed images are substituted.
However, detecting the thinning state of the photoconductive layer is quite difficult.
In other words, when the numbers of the formed images exceed a predetermined number it is judged that the life of the photoconductor is run out and the photoconductor should be replaced with a new one.
The thinning degree of the photoconductive layer of the photoconductor varies mainly according to a density of a document image, a level of an exposure lamp and a size of an image to be formed even if only one image is formed from the document.
For the above-mentioned known image forming apparatus, the life of the photoconductor is determined on a basis of only the numbers of the formed images regardless of the image density or the image size. As a result, there is found a great difference between an actual life and the determined life of the photoconductor.
Furthermore, the photoconductor in a sufficiently usable state may be wastefully replaced for the photoconductor which is hardly deteriorated since the numbers of the formed images up to a replacement of the photoconductor is set on an assumption that the photoconductor is used in a severe state in order to prevent a use of the deteriorated photoconductor.
In a case that the damaged photoconductor is not determined as running out of the life, the qualities of the formed images will be lowered.
In addition, in a case of using the scolotron charger, the capacitance of the photoconductive layer is increased due to the thinning of the photoconductive layer since the amount of the charges on the photoconductor is increased in order to maintain a constant surface potential.
When the image exposure is performed on the photoconductor having a thinning photoconductive layer at the same exposure level as that of the exposure on the photoconductor without any thinning of the photoconductive layer, the charges on the photoconductor cannot be sufficiently canceled and the formed images by using the photoconductor having the thinning photoconductive layer are darken comparing a normal brightness of the formed images by using the photoconductor without any thinning of the photoconductive layer.